5 Answers
5

It did exist but no one is sure what it was. The making of such was split between different orders and each only knew how to make the next step in the chain. It was delivered via tubes and could be "thrown" towards the enemy. Some of those were man-portable, other were ship bound. Sometimes, you could find it in jars.

The best guess is that it was a petroleum compound but the exact formula is lost.

Greek fire was used by the Byzantines, often on their war boats, as an incendiary to enemy vessels. The formula likely consisted of some mixture of naptha, sulfur, and niter among other compounds. This was basically a Byzantine napalm and was effective at sea because the fire could continue to burn (due to the underlying subtances being highly flammable and not strictly water soluble) even with some water hitting it.

The byzantine war boats fired the ignited substance under pressure allowing some distance to be reached.

The greek fire certainly did exist, there is a sufficient number of evidences supporting that, including drawings like this one:

It was apparently some flammable substance that would be hurled towards enemy boats to ignite them, definitely not an explosive however. The exact formula is lost so you will only find some guesses as to what it might have been.